Wales is home to a large number of barn conversions. The old stone barns look really attractive in their rural setting and the materials used create the local vernacular. The old stone walls are the main feature of the barns and so the planners are really keen on keeping up appearances and so insist on minimal changes to the exterior.
All of this sounds fine, but here we hit a problem in upgrading these lovely old buildings from being an agricultural one to a domestic inhabited one. Modern building regulations state that you need to bring the thermal insulation of the building up to 21st century standards. Again this sounds fine, but the trouble begins with our dependence on computer modelling and building ‘science’. We have created a system where we believe that thick solid walls perform really badly compared to modern cavity walls with inbuilt insulation. Recent tests from the BRE (Building Research Establishment – the be all and end all of building science) show clearly that solid walls do not all perform the same. It has taken to 2011 to find this out!!
At present all solid walls are treated as having a U value of 2.1. A U value is the measure of its thermal performance and the lower it is the better. Modern houses are allegedly built with walls of a value of 0.3. Barn conversions need to achieve 0.35 (why heaven knows!). However this BRE research now says that solid walls have a U value range of between 0.7 and 1.6. Nowhere near the 2.1 that appears in all the architects, building control and EPC software. So effectively we are making old buildings out-perform new modern buildings purely by using the wrong figure in the computer software.
This could be taken as a positive thing. Your barn conversion out performing a modern house. Marvellous. However, in order to get a figure of 0.35 from a ‘starting point’ of 2.1 takes a lot of insulation. Space in barns can be tight and so people are almost being forced to choose high performance insulation (phenolic boards, Expanded Polystyrene etc.) These boards are placed in a frame built off of the irregular walls and hence create four things.
1. A gap behind the insulation which is ideal for rodents, bugs etc
2. An thermal isolation between the old walls (with their thermal mass) and the modern living space
3. A non-breathing barrier between the internal plaster / insulation walls and the stone wall
4. A visual barrier between the inside and the internal face of the stone wall
So not only do you have a warm space for mice and other things to live in and nibble away at cables etc, you have effectively isolated the stone wall and all its thermal mass from the living space. So you cannot see what is happening there, you cannot see any moisture or damp problems, you have to manage the warm moist air in a mechanical way rather than relying on the breathable wall to do the work for you free of charge and the wall is being put in danger by being colder and hence more susceptible to interstitial damp.
This isolation of the wall purely to get the correct computer derived U value that is based on a false premise can therefore cause many more problems than it solves. The system and our reliance on factually incorrect assumptions built into software is therefore a major issue that needs to be addressed. Maybe the new Building Regulations in Wales will take this in account!!???
We think that ideally the walls of these old barns are not isolated by non-breathing insulation, rather that they be insulated with a material that adjoins the wall to maintain the solid nature of it and also maintains breathability. There are a wide variety of materials that can achieve this. Wood fibre insulation, hemp/lime plaster, insOwall insulating plaster, Sheep’s wool boards etc. etc.
Even if we keep the 2.1 value (although I see no reason why we should) it is possible to get to 0.35, or to a figure for the whole house that takes some of the pressure off of the walls, by using natural materials that will work in harmony with an old building. For example increasing the thermal performance of the floors, doors, windows and ceiling will allow for thinner insulation on the walls. We really advise that you look at your project holistically and really aim to maintain the true nature of the building by using the right materials that will deliver you a long lasting, healthy, comfortable and cheap to maintain home.